Female Grass Wrens adjust brooding effort according to ambient temperature, male feeding contribution and nest placement

Abstract

The keystone of life‐history evolution theory proposes a trade‐off between current reproductive effort and future survival. Brooding is an essential and often overlooked component of parental behaviour of altricial birds intended to transfer heat to nestlings. Brooding under challenging environmental conditions can be energetically demanding and may adversely affect adult survival and future reproduction. We provide the first detailed description of the brooding behaviour of a south temperate grassland specialist: the Grass Wren Cistothorus platensis. In Grass Wrens, both sexes collaborate in parental care; however, only females brood the nestlings. We examined the relationship of ambient temperature, male feeding contribution, nestling age, nest size and nest placement in determining brooding behaviour. Additionally, we evaluated the effects of brooding behaviour on nestling body condition near fledging and female apparent survival. Brooding effort decreased with nestling age. Females increased their brooding effort at low ambient temperature and when males delivered more food. At lower ambient temperatures, females increased brooding effort in nests placed further from plant edge, with taller plant heights and well‐concealed substrates. However, there was no relationship between brooding effort and nest size. Neither female investment in brooding nor the number of nestlings produced per female was associated with nestling body condition near fledging or female annual apparent survival. Our results identify several factors that shape brooding behaviour in this south temperate passerine, suggesting that females favour nestling thermal conditions under adverse weather conditions.